The present invention generally relates to printed circuit boards, and more particularly to backpanels, or those which have system busses embedded therein.
Prior art backpanels often carry busses for more than one logic family, when those logic families are present in the computer system served. For example, a computer system may contain emitter-coupled logic (ECL) circuits and transistor-transistor logic (TTL) circuits which require separate busses. ECL busses require approximately 50 ohms nominal impedance, while TTL busses require approximately 76 ohms nominal impedance. Because of bus size requirements, prior art backpanels, which often use "micro-strip" construction (ground and power layers in between the signal layers), have separate signal layers for each logic family, thus adding volume and expense.
The prior art does not contemplate combining ECL and TTL signals on the same signal layer since, inter alia, the ECL tracings would be too large compared to the TTL tracings (there would not be enough room in a signal layer of a prior art backpanel for both logic families to reside).